doc: update install from source instructions for go1.5.3

Fixes #14020

Change-Id: I454c2613912a7efcb464c6e6f3ac2e0ec89fb719
Reviewed-on: https://go-review.googlesource.com/18750Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

doc/install-source.html

@@ -184,7 +184,7 @@ Then clone the repository and check out the latest release tag:</p>
 <pre>
 $ git clone https://go.googlesource.com/go
 $ cd go
-$ git checkout go1.5.2
+$ git checkout go1.5.3
 </pre>
 
 <h2 id="head">(Optional) Switch to the master branch</h2>
@@ -363,7 +363,7 @@ New releases are announced on the
 <a href="//groups.google.com/group/golang-announce">golang-announce</a>
 mailing list.
 Each announcement mentions the latest release tag, for instance,
-<code>go1.5.2</code>.
+<code>go1.5.3</code>.
 </p>
 
 <p>